The invention relates to a laser oscillator and a method of controlling it. The laser oscillator comprises a laser unit and a control unit for applying at least one control signal to at least the control input of the laser unit on the basis of a desired (i.e., predetermined) value of at least a physical quantity (i.e., characteristic) of the laser light.
A laser oscillator of this type is known from U.S. Pat. No. 4,509,130. Such laser oscillators are used, for example, in transmitters or receivers for coherent optical transmission systems, in spectroscopy equipment or in test equipment for optoelectric converters.
To transport a baseband signal via a glass fibre in coherent optical transmission systems, a light signal coming from a transmitting laser can be amplitude, frequency or phase modulated by the baseband signal before the light signal is fed to the glass fibre. To demodulate the light signal at a receiver with the aid of current electronic components, it is necessary to convert the light signal, which has a very high frequency (for example, 10.sup.14 Hz) to a much lower intermediate frequency, for example, 10.sup.9 Hz. For this purpose, the received light signal is combined in the receiver with a local laser-generated light signal with the aid of an optical coupling element. This combination provides an optical signal having amplitude variations due to interference between the two input signals of the coupling element. These amplitude variations have a frequency which is equal to the difference frequency between the frequency of the received light signal and that of the locally generated light signal. A photodiode is then used for converting the optical amplitude variations into an electrically processable intermediate-frequency signal.
To simultaneously transport more than a single light signal via a glass fibre, lasers which are tunable over a large frequency range (for example, 500 GHz) are used in both the transmitter and the receiver. As a result, more transmitters and receivers may communicate via the same glass fibre without causing interference to one another.
In prior-art laser oscillators, the physical quantity of the laser light is the frequency of the laser light. Another possible physical quantity is the power of the laser light.
In prior-an laser oscillators, the laser unit has two control inputs, a first control input for which determines the current flowing through the laser diode and a second control input determines the temperature of the laser diode. The control unit comprises a Table in the form of a ROM or a RAM in which values for the control signals are stored as functions of the desired value of the physical quantity (in this case, the frequency of the laser light).
As a result, the value of the physical quantity of the laser light may be modified in steps. With a predetermined range of values for the physical quantity of the laser light, the smallest possible step size with which the value of the physical quantity can be modified is determined by the size of the Table. A disadvantage of prior-art laser oscillators is that when the physical quantity of the laser light has a large range of values and a small step size is desired, the Table must be rather large. Thus, the RAM or ROM is required to have a large memory capacity.